Are you struggling with pesky weeds? Post-emergent herbicides can be the solution you’re looking for. To get rid of dandelions, crabgrass, and other common lawn invaders, you should know when to apply post-emergent herbicides. Consider the weed type, air and soil temperatures, the time of day, and weather conditions to make the application worthwhile.
What are post-emergent herbicides?
Post-emergent herbicides are chemicals designed to kill established and visible weeds. They’re often applied to weeds that are difficult to eliminate manually or with organic weed killers.
The type of post-emergent you choose should align with the weeds you’re trying to eliminate and your long-term lawn care goals. The best post-emergent for you might not be the best for your neighbor. Your choice also will influence how you apply the product.
The best time to apply post-emergent herbicides
When should you apply a post-emergent herbicide? The short answer is whenever you see weeds in your yard. If you missed your pre-emergent application window or the product didn’t do its job, post-emergent is your next best choice. Here is when you should consider applying post-emergents:
Apply post-emergents according to weed type
Knowing what weeds grow in your yard (and when) determines when you apply post-emergents.
Weeds are generally categorized as annual, biennial, or perennial:
- Summer annuals: Typically sprout in early to mid-spring, when temperatures warm up and the soil is moderately moist (not too dry or saturated). They develop and produce seeds during the summer, and die at first frost. The exact timing depends on the local climate.
- Winter annuals: Emerge in early to mid-fall, establishing roots and foliage. As they grow into spring, they mature, flower, set seed, and eventually die from heat.
- Biennial: Complete their life cycle over two years. In the first year, they develop roots, stems, and leaves and form a low-growing rosette (a circular arrangement of leaves). In the second year, they grow a flowering stalk, produce flowers, set seeds, and die.
- Perennial: Live for more than two years, sometimes indefinitely. Their extensive root systems enable them to survive from year to year.
Summer and winter annual weeds
Harry Rose | Flickr | CC BY 2.0
The University of Florida Extension says the best time to apply post-emergent is when the weeds are small but actively growing (early to mid-spring and early to mid-fall). At this stage, they absorb weed killer more easily.
Once the plants mature, things get more challenging, adds the University of Minnesota Extension: “Larger weeds have more growing points than smaller seedlings, which helps them tolerate and escape an herbicide application.” Keep weeds under 3 or 4 inches tall, or they’ll be much harder to kill.
Common summer and winter annual weeds:
- Chickweed (winter annual)
- Crabgrass (summer annual)
- Henbit (winter annual)
- Foxtail (summer annual)
- Shepherd’s purse (winter annual)
- Spurge (summer annual)
Biennial weeds
Biennials follow a two-year cycle. In the first year, they develop roots, stems, and leaves and form a low-growing rosette (a circular arrangement of leaves). In the second year, they grow a stalk, produce flowers, set seeds, and die.
If you didn’t apply pre-emergent in time, and the plants have started forming rosettes, treat them with a post-emergent from late winter to early spring (usually from February to April). This prevents flowering, seed production, and second-year problems.
If you missed that window, you can also apply post-emergent in early to mid-spring, as long as the weeds are still in their rosette stage. Consider your local climate.
Perennial weeds
New York State IPM Program at Cornell University | Flickr | CC BY 2.0
Perennials are active in spring, summer, and fall.
- Spring: Existing plants emerge from dormancy and begin their active growth as soon as temperatures rise. This is also when new plants sprout. Apply post-emergent in early to mid-spring.
- Summer: They develop foliage, produce flowers, and set seeds. During this time, they become impressively vigorous. Apply post-emergent in early summer, during their active growth period and before they potentially enter dormancy due to extreme heat.
- Fall: They naturally slow their above-ground growth and prepare for winter dormancy and the next growing season. As they transfer nutrients to their roots, an early to mid-fall post-emergent application ensures the herbicide gets transferred as well. Apply the treatment at least three to four weeks before the first frost.
Common perennial weeds:
- Bindweed
- Clover
- Creeping Charlie (or ground ivy)
- Canada thistle
- Dandelion
- Plantain
Apply post-emergents according to air and soil temperatures
The best time to use post-emergent herbicide is when daytime temperatures fall between 65 and 85 degrees Fahrenheit and soil temperatures are within the 55 to 65-degree range. This is the same for spring and fall applications.
Air temperatures below 65 degrees will significantly slow the weed control process. In colder weather, weeds absorb and process herbicides more slowly, making them less effective and extending the time it takes to see results.
Consider the following general application guidelines:
- The South
- Spring: February to April
- Fall: September to November
- Central region
- Spring: March to May
- Fall: August to October
- The North
- Spring: April to June
- Fall: August to September
Apply post-emergents according to time of day
The best time of day to apply your post-emergent is late morning to early afternoon. During this window, temperatures are usually warm and sunlight is at its peak, boosting herbicide uptake by the weeds.
Avoid applying herbicides very early; morning dew can dilute or wash away the herbicide before it’s fully absorbed. Likewise, avoid evening applications, as weeds slow their growth in preparation for night. Dew also can happen later in the day, again interfering with absorption.
Apply post-emergents according to weather conditions
To get the most out of your post-emergent application, consider these weather conditions:
- Dry weather: Check that there’s no chance of rain for 24 to 48 hours after application. This helps the herbicide dry and penetrate the weeds without getting washed away. Rain can dilute the herbicide and reduce its effectiveness.
- Calm day: Light winds reduce the risk of herbicide drift, which can damage non-target plants and reduce the effectiveness of the treatment.
- Low humidity: Moderate humidity helps ensure proper drying and absorption. Excessive humidity can make this more challenging.
FAQ about when to apply post-emergent herbicides
Should I apply post-emergent before or after overseeding my lawn?
Apply post-emergent herbicides several weeks before you overseed the lawn. This allows the herbicide to break down, minimizing the risk to new seedlings. Weeds also compete with newly seeded grass, so eliminating them beforehand is crucial.
How does drought affect the application and effectiveness of post-emergents?
Post-emergents are most effective when weeds are healthy and actively growing. In drought conditions, weeds experience stress and may stop growing. They also won’t absorb the herbicide as they should, reducing its effectiveness.
Plus, many post-emergents require moisture for activation and absorption. Without rainfall or irrigation (in case of local restrictions), this won’t happen.
How many times can I re-apply post-emergent?
Generally, you can apply most post-emergent herbicides once or twice per growing season. If done correctly and under the right conditions, this controls most weeds. Refer to the label for how often to reapply, which is often every two to four weeks.
Check if the first herbicide application worked before applying more. If you’re not seeing results, it might be due to incorrect application or interfering environmental conditions. Avoid using too much herbicide to prevent weed resistance and damage to your lawn and nearby plants.
A pro can rid your lawn of weeds
DIY lawn care isn’t always the best option for everyone. Even if post-emergent herbicides are generally easy to use, you may have concerns about application or time limitations. In that case, a weed control expert can assist you.
For a thriving lawn, Lawn Love can also connect you with a local lawn care professional for aeration, dethatching, fertilization, and mowing.
Main Image Credit: Shutterstock
